1. Field of the Invention
This invention relates to a method for producing a sintered metallic product, more particularly to a method in which a green compact of a metallic powder is constrained by using a rigid mold during sintering. The density of a sintered billet is denser than that of a sintered billet free of using a rigid mold.
2. Description of the Related Art
Sintering is a process of bonding together distinct particles by the steps such as mixing powers, compacting and sintering. A green compact is formed for easy handling after compacting and is sintered at a temperature below the melting point of powder materials under normal pressure. Generally, the density of a sintered billet is increased as sintering time is increased. It is because the free energy should be decreased by the shrinkage of voids in the sintered billets to decrease the surface energy. However, the sintering behavior is changed, when two kinds of elemental powders, which have a large difference in diffusivity such as aluminum and titanium, are used as materials. A large amount of cavities are formed due to the Kirkendall effect. The Kirkendall effect is a phenomena occurred when two different kinds of elemental powder particles having large difference in diffusivity are interdiffused during sintering. The elemental powder particles having higher diffusivity will diffuse more into the other particles having lower diffusivity. Cavities or pores are formed in the region of particles having higher diffusivity. Finally, a loose, swelling and cracking sintered product is obtained.
Therefore, there is a need to avoid or suppress the formation of voids resulting from the Kirkendall effect when sintering a green compact of aluminum and titanium powders. An effective process, i.e. a hot isostatic pressing (HIP), has long been adapted for the sintering of aluminum and titanium powders. In this process, the so-called "canning " process is first carried out to load and seal the green compact of the metal powders into a flexible container. The loaded container is then pressed by applying uniformly a gas (or liquid) pressure to the container, and is sintered at an elevated temperature. This process can effectively suppress the generation of voids during sintering. However, the operation of this process is very complicated and the capital and operating costs are generally high.